There is a known conventional method in which a programmable logic controller is duplicated to improve reliability, with one of the duplicate programmable logic controllers functioning as a control system and the other functioning as a standby system.
With a duplex system in which a programmable logic controller is a master station that controls a slave station, in order for the programmable logic controller that has been switched from the standby system to the control system to continue controlling the slave station, the programmable logic controller that has been switched from the standby system to the control system needs to control the slave station by using the same data as the data that the programmable logic controller that was the control system received from the slave station.
One known method for causing a programmable logic controller that is the standby system to hold the same data as the control system is to transmit the same data from the slave station to the control system and the standby system. In addition, Patent Literature 1 discloses a method for periodically transferring data from the control system to the standby system.